6.设x1,x2···x16为来自总体N(1,4)的样本,x为样本均-|||-值,则 overline (x)- ()()-|||-A (16,dfrac (1)(4))-|||-B (16,dfrac (1)(16))-|||-C (1,dfrac (1)(16))-|||-D (1,dfrac (1)(4))